Austenitic heat resistant alloy

ABSTRACT

There is provided an austenitic heat resistant alloy having a chemical composition that contains, in mass percent: C: 0.03 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.010% or less, Cr: 13.0 to 30.0%, Ni: 25.0 to 45.0%, Al: 2.5 to 4.5%, Nb: 0.01 to 2.00%, N: 0.05% or less, Ti: 0 to 0.20%, W: 0 to 6.0%, Mo: 0 to 4.0%, Zr: 0 to 0.10%, B: 0 to 0.0100%, Cu: 0 to 5.0%, REM: 0 to 0.10%, Ca: 0 to 0.050%, Mg: 0 to 0.050%, and the balance: Fe and impurities.

TECHNICAL FIELD

The present invention relates to an austenitic heat resistant alloy.

BACKGROUND ART

Olefins (C_(n)H₂n) such as ethylene (C₂H₄) are produced by subjectinghydrocarbons (naphtha, natural gas, ethane, etc.) to heat decomposition.Specifically, olefinic hydrocarbons (ethylene, propylene, etc.) areobtained by supplying hydrocarbons and steam to an inside of a pipe thatis installed in a reactor and made of a high Cr-high Ni alloy, typically25Cr-25Ni alloys or 25Cr-38Ni alloys, or is made of a stainless steel,typically SUS304 or the like, and by adding heat from an outer surfaceof the pipe, so that a heat decomposition reaction of the hydrocarbonsoccurs on an inner surface of the pipe.

As a demand of synthetic resins has increased in recent years, atendency of a higher temperature has become stronger in use conditionsof a pyrolytic furnace pipe for ethylene plant, from a viewpoint ofincreasing an ethylene yield. The inner surface of such a pyrolyticfurnace pipe is exposed to a carburizing atmosphere, and thus there is ademand for a heat resistant material that is excellent in hightemperature strength and carburization resistance properties.

Moreover, as carburization proceeds, a phenomenon called coking in whichcarbon precipitates on the inner surface of the pyrolytic furnace pipeoccurs during operation. As a precipitation amount in the cokingincreases, a harmful effect on the operation, such as an increase inpressure loss and a decrease in heating efficiency, arises. Therefore,in a practical operation, oxidization and removal of the precipitatingcarbon by supplying air and steam, what is called a decoking operation,are performed periodically, which however raises a major problem such asan operation stop during the decoking operation and an increase innumber of work person-hours.

Prior art includes developments of materials each having improvedcarburization resistance properties. For example, JP2001-40443A (PatentDocument 1) proposes a Ni-based heat resistant alloy that is excellentin hot workability, weldability, and carburization resistanceproperties. However, a Ni-based alloy is difficult to produce because aγ′ phase, which is a brittle phase, precipitates at high temperature,narrowing a temperature range that allows hot working.

Hence, there is a development of a Fe-based austenitic stainless steelfor improvement of the hot workability. For example, WO 2017/119415(Patent Document 2) proposes an austenitic heat resistant alloy thatkeeps a high creep strength and a high toughness even in ahigh-temperature environment.

LIST OF PRIOR ART DOCUMENTS Patent Document

Patent Document 1: JP 2001-40443A

Patent Document 2: WO 2017/119415

SUMMARY OF INVENTION Technical Problem

The austenitic heat resistant alloy described in Patent Document 2 formsan alumina layer on its surface while being used at high temperature,which not only provides high corrosion resistances but also allows theaustenitic heat resistant alloy to have a long-term high-temperaturestrength and an excellent toughness. However, Patent Document 2 has nosufficient investigation on the carburization resistance properties,leaving room for improvement.

The present invention has an objective to provide an austenitic heatresistant alloy that keeps a high creep strength and excellentcarburization resistance properties even in its use in a hightemperature environment.

Solution to Problem

The present invention is made to solve the problem described above, andthe gist of the present invention is the following austenitic heatresistant alloy.

(1) An austenitic heat resistant alloy having a chemical compositionconsisting of, in mass percent:

C: 0.03 to 0.25%;

Si: 0.01 to 2.0%;

Mn: 0.10 to 0.50%;

P: 0.030% or less;

S: 0.010% or less;

Cr: 13.0 to 30.0%;

Ni: 25.0 to 45.0%;

Al: 2.5 to 4.5%;

Nb: 0.05 to 2.00%;

N: 0.05% or less;

Ti: 0 to 0.20%;

W: 0 to 6.0%;

Mo: 0 to 4.0%;

Zr: 0 to 0.10%;

B: 0 to 0.0100%;

Cu: 0 to 5.0%;

REM: 0 to 0.10%;

Ca: 0 to 0.050%;

Mg: 0 to 0.050%; and

the balance: Fe and impurities.

(2) The austenitic heat resistant alloy according to the above (1),wherein the chemical composition contains, in mass percent, B: 0.0010 to0.0100%.

(3) The austenitic heat resistant alloy according to the above (1) or(2), wherein in a case where the alloy is heated in the atmospherecontaining steam at 900° C. for 20 hours and subsequently heated in anH₂—CH₄—CO₂ atmosphere at 1100° C. for 96 hours, a continuous aluminalayer having a thickness ranging from 0.5 to 15 μm is formed on asurface of the alloy.

(4) The austenitic heat resistant alloy according to the above (3),wherein in the case where the alloy is heated in the atmospherecontaining steam at 900° C. for 20 hours and subsequently heated in theH₂—CH₄—CO₂ atmosphere at 1100° C. for 96 hours, a layer having aCr—Mn-based spinel structure formed on the alumina layer has a thicknessof 5 μm or less.

Advantageous Effects of Invention

According to the present invention, an austenitic heat resistant alloythat keeps a high creep strength and excellent carburization resistanceproperties even in its use in a high temperature environment can beobtained.

DESCRIPTION OF EMBODIMENTS

The present inventors conducted investigations and studies aboutcarburization resistance properties of an austenitic heat resistantalloy in a high-temperature environment at 1000° C. or more(hereinafter, referred to simply as “high temperature environment”), andobtained the following findings.

Carburization resistance properties at high temperature can be kept byforming a continuous alumina layer on a surface of a base metal. Theformation of the alumina layer is promoted by presence of Cr. Thiseffect is called the third element effect (TEE) of Cr. In a very earlystage of oxidation, Cr is preferentially oxidized on the surface of thebase metal, forming a chromia layer.

This consumes oxygen in the surface of the base metal, decreasing anoxygen partial pressure. As a result, Al does not undergo internaloxidation but forms the continuous alumina layer in proximity to thesurface. Afterward, oxygen used by the chromia layer is taken by thealumina layer, by which a protective layer made only of alumina iseventually formed. Therefore, to form a continuous alumina layer havinga protectability, Cr needs to be contained at a certain content or more.

Here, in a case where a heat resistant alloy is used in a form of apyrolytic furnace pipe, it is not possible to completely prevent theoccurrence of coking. This requires a decoking operation to be performedperiodically. At that time, the decoking removes even the alumina layerformed on the surface of the base metal. Therefore, when the heatresistant alloy is reused in the high temperature environment, it isdesirable that the continuous alumina layer recovers itself immediately.

However, if the layer having the “Cr—Mn-based spinel structure” (in thefollowing description, also referred to as “Cr—Mn spinel layer”) isproduced excessively in the use, Cr in an outer layer of the base metalruns short. This restrains the TEE as a period of the use increases,which causes Al to undergo internal oxidation, forming discontinuousalumina layers on the surface. As a result, the alumina becomes unableto fulfill a function as the protective layer.

That is, in order to keep self-recovery properties of the alumina layerfor a long time, it is necessary to restrain the formation of the Cr—Mnspinel layer on the surface of the base metal. To this end, it isnecessary to reduce a content of Mn in the base metal.

The present invention is made based on the findings described above.Requirements of the present invention will be described below in detail.

1. Chemical Composition

The reasons for limiting contents of elements are as described below. Inthe following description, the symbol “%” for the contents means“percent by mass.”

C: 0.03 to 0.25%

C (carbon) forms carbides, increasing the creep strength. Specifically,C binds with alloying elements to form fine carbides in crystal grainboundaries and grains in the use in the high-temperature environment.The fine carbides increase deformation resistance, thereby increasingthe creep strength. If a content of C is excessively low, this effect isnot obtained. In contrast, if the content of C is excessively high, alarge number of coarse eutectic carbides are formed in a solidificationmicro-structure of the heat resistant alloy after casting. The eutecticcarbides remain coarse in the micro-structure even after solutiontreatment, thus decreasing a toughness of the heat resistant alloy. Inaddition, the remaining coarse eutectic carbides make it difficult forthe fine carbides to precipitate in the use in the high-temperatureenvironment, decreasing the creep strength. Accordingly, the content ofC is to range from 0.03 to 0.25%. A lower limit of the content of C ispreferably 0.04%, more preferably 0.05%. An upper limit of the contentof C is preferably 0.23%, more preferably 0.20%.

Si: 0.01 to 2.0%

Silicon (Si) deoxidizes the heat resistant alloy. In addition, Siincreases corrosion resistances (oxidation resistance and steamoxidation resistance) of the heat resistant alloy. Si is an element thatis contained unavoidably, but in a case where the deoxidation can beperformed sufficiently by other elements, a content of Si may be as lowas possible. In contrast, if the content of Si is excessively high, thehot workability is decreased. Accordingly, the content of Si is to rangefrom 0.01 to 2.0%. A lower limit of the content of Si is preferably0.02%, more preferably 0.03%. An upper limit of the content of Si ispreferably 1.0%, more preferably 0.3%.

Mn: 0.10 to 0.50%

Manganese (Mn) binds with S contained in the heat resistant alloy toform MnS, increasing the hot workability of the heat resistant alloy.However, if a content of Mn is excessively high, the heat resistantalloy becomes excessively hard, decreasing in the hot workability andthe weldability. In addition, the excessively high content of Mn causesthe production of the Cr—Mn spinel layer described above, which inhibitsthe TEE, inhibiting uniform formation of the alumina layer. Accordingly,the content of Mn is to range from 0.10 to 0.50%. An upper limit of thecontent of Mn is preferably 0.40%, more preferably 0.30%, still morepreferably 0.20%.

P: 0.030% or less

Phosphorus (P) is an impurity. P decreases the weldability and the hotworkability of the heat resistant alloy. Accordingly, the content of Pis to be 0.030% or less. The content of P is preferably as low aspossible.

S: 0.010% or less

Sulfur (S) is an impurity. S decreases the weldability and the hotworkability of the heat resistant alloy. Accordingly, a content of S isto be 0.010% or less. The content of S is preferably as low as possible.

Cr: 13.0 to 30.0%

Chromium (Cr) increases corrosion resistances (oxidation resistance,steam oxidation resistance, etc.) of the heat resistant alloy in thehigh temperature environment. In addition, Cr brings about the TEE,promoting the uniform formation of the alumina layer. However, if acontent of Cr is excessively high, the formation of the chromia layerbecomes predominant, and the formation of the alumina layer is ratherinhibited. Accordingly, the content of Cr is to range from 13.0 to30.0%. A lower limit of the content of Cr is preferably 15.0%. An upperlimit of the content of Cr is preferably 25.0%, and more preferably20.0%.

Ni: 25.0 to 45.0%

Nickel (Ni) stabilizes austenite. In addition, Ni binds with Al to formfine NiAl, increasing the creep strength. Moreover, Ni has an effect ofincreasing the corrosion resistances of the heat resistant alloy as wellas an effect of increasing the carburization resistance properties bydecreasing a diffusion velocity of C in the steel. If a content of Ni isexcessively low, these effects are not obtained. In contrast, if thecontent of Ni is excessively high, these effects level off, andfurthermore, the hot workability is decreased. In addition, theexcessively high content of Ni increases a raw-material cost.Accordingly, the content of Ni is to range from 25.0 to 45.0%. A lowerlimit of the content of Ni is preferably 30.0%. An upper limit of thecontent of Ni is preferably 40.0%, more preferably 35.0%.

Al: 2.5 to 4.5%

Aluminum (Al) forms the alumina layer, which is excellent in thecarburization resistance properties, in the use in the high temperatureenvironment. In addition, Al binds with Ni to form the fine NiAl,increasing the creep strength. If a content of Al is excessively low,these effects are not obtained. In contrast, if the content of Al isexcessively high, a structural stability is decreased, and a strength isdecreased. Accordingly, the content of Al is to range from 2.5 to 4.5%.A lower limit of the content of Al is preferably 2.8%, more preferably3.0%. An upper limit of the content of Al is preferably 3.8%. In theaustenitic heat resistant alloy according to the present invention, thecontent of Al means a total amount of Al contained in the alloy.

Nb: 0.05 to 2.00%

Niobium (Nb) forms intermetallic compounds (Laves phase and Ni3Nb phase)to be precipitation strengthening phases, so as to bring aboutprecipitation strengthening in the crystal grain boundaries and thegrains, increasing the creep strength of the heat resistant alloy. Incontrast, if a content of Nb is excessively high, the intermetalliccompounds are produced excessively, decreasing the toughness and the hotworkability of the alloy. The excessively high content of Nbadditionally decreases a toughness after long-time aging. Accordingly,the content of Nb is to range from 0.05 to 2.00%. A lower limit of thecontent of Nb is preferably 0.50%, more preferably 0.80%. An upper limitof the content of Nb is preferably 1.20%, more preferably 1.00%.

N: 0.05% or less

Nitrogen (N) stabilizes austenite and is unavoidably contained through anormal solution process. However, if a content of N is excessively high,coarse carbo-nitrides are formed and remain undissolved even after thesolution treatment, decreasing the toughness of the alloy. Accordingly,the content of N is to be 0.05% or less. An upper limit of the contentof N is preferably 0.01%.

Ti: 0 to 0.20%

Titanium (Ti) forms the intermetallic compounds (Laves phase and Ni₃Tiphase) to be the precipitation strengthening phases, so as to bringabout the precipitation strengthening, increasing the creep strength.Therefore, Ti may be contained as necessary. However, if a content of Tiis excessively high, the intermetallic compounds are producedexcessively, decreasing a high temperature ductility and the hotworkability. The excessively high content of Ti additionally decreasesthe toughness after long-time aging. Accordingly, the content of Ti isto be 0.20% or less. An upper limit of the content of Ti is preferably0.15%, more preferably 0.10%. Note that the content of Ti is preferably0.03% or more in a case where an intention is to obtain the aboveeffect.

W: 0 to 6.0%

Tungsten (W) is dissolved in the austenite being a parent phase(matrix), bringing about solid-solution strengthening to increase thecreep strength through. In addition, W forms Laves phases in the crystalgrain boundaries and the grains, bringing about the precipitationstrengthening to increase the creep strength. Therefore, W may becontained as necessary. However, if a content of W is excessively high,the Laves phases are produced excessively, decreasing the hightemperature ductility, the hot workability, and the toughness.Accordingly, the content of W is to be 6.0% or less. An upper limit ofthe content of W is preferably 5.5%, more preferably 5.0%. Note that thecontent of W is preferably 0.005% or more, and more preferably 0.01% ormore in a case where an intention is to obtain the above effect.

Mo: 0 to 4.0%

Molybdenum (Mo) is dissolved in the austenite being the parent phase,bringing about the solid-solution strengthening to increase the creepstrength through. In addition, Mo forms the Laves phases in the crystalgrain boundaries and the grains, bringing about the precipitationstrengthening to increase the creep strength. Therefore, Mo may becontained as necessary. However, if a content of Mo is excessively high,the Laves phases are produced excessively, decreasing the hightemperature ductility, the hot workability, and the toughness.Accordingly, the content of Mo is to be 4.0% or less. An upper limit ofthe content of Mo is preferably 3.5%, more preferably 3.0%. Note thatthe content of Mo is preferably 0.005% or more, and more preferably0.01% or more in a case where an intention is to obtain the aboveeffect.

Zr: 0 to 0.10%

Zirconium (Zr) brings about grain-boundary strengthening, increasing thecreep strength. Therefore, Zr may be contained as necessary. However, ifa content of Zr is excessively high, the weldability and the hotworkability of the heat resistant alloy are decreased. Accordingly, thecontent of Zr is to be 0.10% or less. An upper limit of the content ofZr is preferably 0.06%. Note that the content of Zr is preferably0.0005% or more, and more preferably 0.001% or more in a case where anintention is to obtain the above effect.

B: 0 to 0.0100%

Boron (B) brings about the grain-boundary strengthening, increasing thecreep strength. Therefore, B may be contained as necessary. However, ifa content of B is excessively high, the weldability is decreased.Accordingly, the content of B is to be 0.0100% or less. An upper limitof the content of B is preferably 0.0050%. Note that the content of B ispreferably 0.0001% or more in a case where an intention is to obtain theabove effect. The lower limit of the content of B is more preferably0.0005%, still more preferably 0.0010%, 0.0020% or more, or 0.0030% ormore.

Cu: 0 to 5.0%

Copper (Cu) promotes the formation of the alumina layer in proximity tothe surface, increasing the corrosion resistances of the heat resistantalloy. Therefore, Cu may be contained as necessary. However, if acontent of Cu is excessively high, the effect levels off, andfurthermore, the high temperature ductility is decreased. Accordingly,the content of Cu is to be 5.0% or less. An upper limit of the contentof Cu is preferably 4.8%, more preferably 4.5%. Note that the content ofCu is preferably 0.05% or more, and more preferably 0.10% or more in acase where an intention is to obtain the above effect.

REM: 0 to 0.10%

Rare earth metal (REM) immobilizes S in a form of its sulfide,increasing the hot workability. In addition, REM forms its oxide,increasing the corrosion resistances, the creep strength, and a creepductility. Therefore, REM may be contained as necessary. However, if acontent of REM is excessively high, inclusions such as the oxide areincreased, decreasing the hot workability and the weldability, andincreasing production costs. Accordingly, the content of REM is to be0.10% or less. An upper limit of the content of REM is preferably 0.09%,more preferably 0.08%. Note that the content of REM is preferably0.0005% or more, and more preferably 0.001% or more in a case where anintention is to obtain the above effect.

Here, in the present invention, REM refers to Sc (scandium), Y(yttrium), and lanthanoids, 17 elements in total, and the content of REMmeans a total content of these elements. In industrial practice, thelanthanoids are added in a form of misch metal.

Ca: 0 to 0.050%

Calcium (Ca) immobilizes S in a form of its sulfide, increasing the hotworkability. Therefore, Ca may be contained as necessary. However, if acontent of Ca is excessively high, the toughness, the ductility, and acleanliness are decreased. Accordingly, the content of Ca is to be0.050% or less. An upper limit of the content of Ca is preferably0.030%, more preferably 0.010%. Note that the content of Ca ispreferably 0.0005% or more in a case where an intention is to obtain theabove effect.

Mg: 0 to 0.050%

Magnesium (Mg) immobilizes S in a form of its sulfide, increasing thehot workability. Therefore, Mg may be contained as necessary. However,if a content of Mg is excessively high, the toughness, the ductility,and the cleanliness are decreased. Accordingly, the content of Mg is tobe 0.050% or less. An upper limit of the content of Mg is preferably0.030%, more preferably 0.010%. Note that the content of Mg ispreferably 0.0005% or more in a case where an intention is to obtain theabove effect.

The balance of the chemical composition described above is Fe andimpurities. The term “impurities” as used herein means components thatare mixed in the alloy in producing the alloy industrially due to rawmaterials such as ores and scraps, and various factors of a producingprocess, and are allowed to be mixed in the alloy within ranges in whichthe impurities have no adverse effect on the present invention.

2. Layer

As described above, it is preferable for the austenitic heat resistantalloy according to the present invention to immediately form thecontinuous alumina layer having a protectability in the high temperatureenvironment. Specifically, in a case where the alloy is heated in theatmosphere containing steam at 900° C. for 20 hours and subsequentlyheated in an H2-CH4-0O2 atmosphere at 1100° C. for 96 hours, it ispreferable that the continuous alumina layer having a thickness rangingfrom 0.5 to 15 μm is formed on the surface of the alloy. Note that thetreatment of heating the alloy in the atmosphere containing steam at900° C. for 20 hours is directed to performing the decoking in advance.

If the thickness of the alumina layer formed by the treatment is lessthan 0.5 μm, the layer is broken in a short time in a high temperaturecarburizing environment, failing to keep the corrosion resistances. Incontrast, if the thickness of the layer is more than 15 μm, the layercannot withstand its internal stress and is prone to form a crack. Notethat whether the alumina layer is continuous is evaluated by observing across section of the layer under a scanning electron microscope (SEM).

Additionally, it is preferable that the formation of the Cr—Mn spinellayer is restrained in the high-temperature environment. Specifically,in the case where the alloy is heated in the atmosphere containing steamat 900° C. for 20 hours and subsequently heated in an H₂—CH₄—CO₂atmosphere at 1100° C. for 96 hours, it is preferable that the thicknessof the layer having the Cr—Mn-based spinel structure formed on thealumina layer is 5 μm or less.

If the thickness of the Cr—Mn spinel layer is more than 5 μm, a Crdepleted zone is produced in the outer layer of the base metal, due towhich the TEE is restrained as a period of the use increases.

3. Producing Method

As an example of a method for producing the austenitic heat resistantalloy according to the present invention, a method for producing analloy pipe will be described. The producing method in the presentembodiment includes a preparation step, a hot forging step, a hotworking step, a cold working step, and a solution heat treatment stepdescribed below. The producing method may further include a scaleremoving step after the solution heat treatment step. The steps will beeach described below.

[Preparation Step]

A molten steel having the chemical composition described above isproduced. The molten steel is subjected to a well-known degassingtreatment as necessary. The molten steel is cast to be produced into astarting material. The starting material may be an ingot made by aningot-making process, or may be a cast piece such as a slab, bloom, andbillet made by a continuous casting process.

[Hot Forging Step]

Hot forging is performed on the cast starting material to produce acylindrical starting material. In the hot forging, its area reductionratio defined by Formula (i) is set at 30% or more.

Area reduction ratio=100−(cross-sectional area of starting materialafter hot working/cross-sectional area of starting material before hotforging)×100 (%)   (i)

[Hot Working Step]

Hot working is performed on the hot-forged cylindrical starting materialto produce an alloy hollow shell. For example, a through hole is formedat a center of the cylindrical starting material by machining. Hotextrusion is performed on the cylindrical starting material with thethrough hole formed to produce the alloy hollow shell. The alloy hollowshell may be produced by performing piercing-rolling on the cylindricalstarting material.

[Cold Working Step]

Cold working is performed on the hot-worked alloy hollow shell toproduce an intermediate material. The cold working is, for example, colddrawing or the like.

In a case where the cold working is performed, its area reduction ratiodefined by Formula (ii) is set at 15% or more.

Area reduction ratio=100−(cross-sectional area of starting materialafter cold working/cross-sectional area of starting material before coldworking)×100 (%)    (ii)

By performing the cold working at the area reduction ratio of 15% ormore, a micro-structure of the base metal becomes close-grained throughrecrystallization in heat treatment, which enables formation of a moreclose-grained alumina layer.

[Solution Heat Treatment Step]

Solution heat treatment is performed on the produced intermediatematerial. By the solution heat treatment, the carbides and theprecipitates included in the intermediate material are dissolved.

In the solution heat treatment, its heat treatment temperature is 1150to 1280° C. If the heat treatment temperature is less than 1150° C., thecarbides and the precipitates are not dissolved sufficiently, and as aresult, the corrosion resistances deteriorate. In contrast, if the heattreatment temperature is excessively high, the crystal grain boundariesare melted. A duration of the solution heat treatment is 1 minute ormore, in which the carbides and the precipitates are dissolved.

[Scale Removing Step]

After the solution heat treatment step, shotblasting may be performed toremove scales formed on the surface. In addition, pickling treatment maybe performed to remove the scales. In this case, the intermediatematerial is immersed in a fluoro-nitric acid at 20 to 40° C. made bymixing 5% hydrofluoric acid and 10% nitric acid, for 2 to 10 minutes.

By the above producing method, the austenitic heat resistant alloyaccording to the present embodiment is produced. The above descriptionis made about the method for producing an alloy pipe, a plate material,but a bar material, a wire rod, or the like may be produced by a similarproducing method.

The present invention will be described below more specifically withreference to examples, but the present invention is not limited to theseexamples.

EXAMPLES

Molten steels having chemical compositions shown in Table 1 wereproduced using a vacuum furnace. The molten steels were used to producecolumn-shaped ingots having an outer diameter of 120 mm. The hot forgingat an area reduction ratio of 60% was performed on the ingots to producerectangular-shaped starting materials. Then, the hot rolling and thecold rolling were performed on the rectangular-shaped starting materialsto produce plate-shaped intermediate materials having a thickness of 1.5mm. In the cold rolling, its area reduction ratio was 50%. Subsequently,the intermediate materials were retained at 1200° C. for 10 minutes andthen water-cooled to be produced into alloy plate materials.

[Table 1]

TABLE 1 Test Chemical composition (mass %, balance: Fe and impurities)No. C Si Mn P S Cr Ni Al Nb N B Others 1 0.10 0.17 0.16 0.012 0.00314.96 34.77 2.79 1.01 0.0019 — — Inventive 2 0.15 0.18 0.31 0.008 0.00613.14 40.66 3.56 0.94 0.0037 — Ti: 0.12 example 3 0.12 0.14 0.22 0.0090.008 28.14 35.10 3.44 0.92 0.0022 0.0031 — 4 0.12 0.11 0.47 0.012 0.00713.24 35.80 2.98 1.20 0.0019 — Ca: 0.0052 5 0.15 0.19 0.11 0.007 0.00721.56 32.80 4.21 0.98 0.0150 0.0078 W: 4.55 6 0.18 0.35 0.35 0.012 0.00928.25 26.21 4.01 0.97 0.0087 0.0006 Mo: 1.98 7 0.11 0.16 0.44 0.0110.007 20.11 36.33 3.80 1.21 0.0069 0.0033 Zr: 0.08 8 0.08 0.27 0.210.011 0.006 15.30 29.55 3.55 1.22 000025 0.0007 Cu: 3.52 9 0.15 0.440.16 0.013 0.005 24.33 30.43 3.52 1.52 0.0033 0.0028 REM: 0.014 10 0.180.19 0.17 0.011 0.005 17.88 30.05 2.81 1.55 0.0021 0.0045 Mg: 0.0020 110.11 0.13 0.44 0.008 0.005 15.33 28.55 4.23 1.05 0.0022 — — 12 0.10 0.110.34 0.020 0.005 18.30 28.94 3.55 1.74 0.0034 — — 13 0.12 0.19 0.210.007 0.001 24.21 38.15 3.24 0.52 0.0340 — W: 0.55 14 0.82 0.21 0.980.013 0.006 23.14 31.64 3.55 1.05 000025 0.0022 — Comparative 15 0.120.14 1.13 0.011 0.004 20.31 35.69 3.14 0.74 0.0021 0.0038 — example 160.14 0.11 1.04 0.012 0.006 20.64 30.27 1.56 1.49 0.0029 0.0038 — 17 0.161.91 0.20 0.021 0.006 25.61 34.5.5 3.21 0.02 0.0022 0.0025 — 18 0.100.16 0.16 0.012 0.001 24.82 39.67 1.99 0.10 0.0086 — — 19 0.10 0.80 0.540.020 0.001 15.05 31.10 2.94 2.20 0.0184 — — 20 0.14 0.15 0.75 0.0080.007 28.64 34.90 3.84 2.50 0.0018 — —

First, from the materials made by subjecting the rectangular-shapedstarting materials to the retention at 1200° C. for 10 minutes and thesubsequent water cooling, round bar creep rupture test specimens eachhaving a diameter of 6 mm and a gage length of 30 mm, which aredescribed in JIS Z 2241(2011), were taken and subjected to the creeprupture test, under conditions of 1000° C. and 10 MPa. The test wasconducted in conformity with JIS Z 2271(2010). When a creep rupture timeof a test specimen was less than 2000 h, the test specimen was rated aspoor (×), when the creep rupture time ranged from 2000 to 3000 h, thetest specimen was rated as good (◯), and when the creep rupture time wasmore than 3000 h, the test specimen was rated as excellent (◯◯).

Next, two of the alloy plate materials were prepared for each testnumber, and the two alloy plate materials were subjected to thecarburizing treatment described below. One of the two alloy platematerials was subjected to carburizing treatment in which the one alloyplate material was heated in an H₂—CH₄—CO₂ atmosphere, at 1100° C., for96 hours (once-treated material).

The once-treated material subjected to the carburizing treatment was cutinto halves in a direction perpendicular to its rolling direction. Oneof the halves was embedded in resin, and its observation surface waspolished, by which a test specimen for observation was fabricated. Then,a kind, a thickness, and a form of the formed layer were observed undera SEM. In addition, a surface of the other of the halves subjected tothe carburizing treatment was subjected to manual dry polishing using#600 abrasive paper, by which scales and the like on the surface wereremoved.

The other of the two alloy plate materials was subjected to a processincluding carburizing treatment in which the other alloy plate materialwas heated in the H₂—CH₄—CO₂ atmosphere, at 1100° C., for 96 hours, andafter the carburizing treatment, heating the other alloy plate materialat 900° C. for 20 hours in the atmosphere containing steam, and theprocess was repeated five times (five-time-treated material).

Then, from a surface of each of the once-treated material and thefive-time-treated material from which scales were removed, a machinedchip for analysis including four 0.5-mm-pitch layers was taken, and aconcentration of C of the machined chip for analysis was measured by thehigh frequency combustion infrared absorption method. From theconcentration, a concentration of C contained in the starting materialis subtracted, by which an increase of C content was determined. In thepresent invention, a case where the increase of C content was 0.3% orless was evaluated as being excellent in the carburization resistanceproperties.

Results of the observation and results of the test are collectivelyshown in Table 2.

TABLE 2 Cr—Mn Increase spinel of C layer Alumina layer content Thick-Thick- (%) Test Creep ness ness five- No. strength (μm) (μm) Form oncetime 1 ∘ — 10 continuous 0.10 0.11 Inventive 2 ∘ — 8 continuous 0.170.16 example 3 ∘∘ — 8 continuous 0.14 0.15 4 ∘ 5 7 continuous 0.22 0.275 ∘∘ — 10 continuous 0.05 0.08 6 ∘ — 8 continuous 0.09 0.08 7 ∘∘ 3 7continuous 0.19 0.23 8 ∘ — 9 continuous 0.11 0.08 9 ∘∘ — 9 continuous0.07 0.08 10 ∘∘ — 7 continuous 0.15 0.15 11 ∘ 4 7 continuous 0.21 0.2212 ∘ — 8 continuous 0.16 0.23 13 ∘∘ — 8 continuous 0.15 0.14 14 x 21 2discontinuous 0.11 0.15 Com- 15 ∘∘ 21 2 discontinuous 0.83 1.25 parative16 ∘∘ 23 — none 1.10 1.72 example 17 x — 7 continuous 0.21 0.25 18 ∘ — 4discontinuous 0.32 0.55 19 ∘ 14 2 discontinuous 0.51 0.89 20 ∘ 19 2discontinuous 0.65 1.06

Referring to Table 2, regarding Test Nos. 1 to 13, their chemicalcompositions satisfied the specification according to the presentinvention, and thus the production of the Cr—Mn spinel layer wasrestrained, and good alumina layers were formed. As a result, theyshowed excellent carburization resistance properties.

In particular, regarding steels except those of Test Nos. 4, 7, and 11,their contents of Mn were reduced to 0.35% or less, and thus theproduction of the Cr—Mn spinel layer was not recognized, and theircarburization resistance properties were consequently more excellentthan others. In addition, regarding Test Nos, 3, 5, 7, 9, 10, and 13, inwhich at least one of B and W is contained, resulted in more excellentcreep strengths than cases where neither B nor W was contained, or thecontent of B or W was insufficient.

In contrast to these, Test Nos. 14 to 20 are comparative examples thatdid not satisfy the specification according to the present invention.Specifically, Test No. 14 had a high content of C, and Test No. 17 had alow content of Nb, and thus Test No. 14 and Test No. 17 resulted in poorcreep strengths.

Regarding Test Nos. 14 to 16, 19, and 20, because their contents of Mnwere high, the Cr—Mn spinel layer was formed, and a Cr depleted zone wasproduced on each outer layer of their base metals, which restrained theTEE, inhibiting the formation of the alumina layer. Regarding Test Nos.16 and 18, their contents of Al were low, resulting in insufficientformation of the alumina layer.

As a result, regarding Test Nos. 14, 15, and 18 to 20, their aluminalayers were formed discontinuously, and regarding Test No. 16, noalumina layer was formed. Therefore, these comparative examples resultedin poor carburization resistance properties for both of theironce-treated materials and five-time-treated materials.

1. An austenitic heat resistant alloy having a chemical compositionconsisting of, in mass percent: C: 0.03 to 0.25%; Si: 0.01 to 2.0%; Mn:0.10 to 0.50%; P: 0.030% or less; S: 0.010% or less; Cr: 13.0 to 30.0%;Ni: 25.0 to 45.0%; Al: 2.5 to 4.5%; Nb: 0.05 to 2.00%; N: 0.05% or less;Ti: 0 to 0.20%; W: 0 to 6.0%; Mo: 0 to 4.0%; Zr: 0 to 0.10%; B: 0 to0.0100%; Cu: 0 to 5.0%; REM: 0 to 0.10%; Ca: 0 to 0.050%; Mg: 0 to0.050%; and the balance: Fe and impurities.
 2. The austenitic heatresistant alloy according to claim 1, wherein the chemical compositioncontains, in mass percent, B: 0.0010 to 0.0100%.
 3. The austenitic heatresistant alloy according to claim 1 or claim 2, wherein in a case wherethe alloy is heated in an atmosphere containing steam at 900° C. for 20hours and subsequently heated in an H₂—CH₄—CO2 atmosphere at 1100° C.for 96 hours, a continuous alumina layer having a thickness ranging from0.5 to 15 μm is formed on a surface of the alloy.
 4. The austenitic heatresistant alloy according to claim 3, wherein in the case where thealloy is heated in the atmosphere containing steam at 900° C. for 20hours and subsequently heated in the H₂—CH₄—CO₂ atmosphere at 1100° C.for 96 hours, a layer having a Cr—Mn-based spinel structure formed onthe alumina layer has a thickness of 5 μm or less.
 5. The austeniticheat resistant alloy according to claim 2, wherein in a case where thealloy is heated in an atmosphere containing steam at 900° C. for 20hours and subsequently heated in an H₂—CH₄—CO₂ atmosphere at 1100° C.for 96 hours, a continuous alumina layer having a thickness ranging from0.5 to 15 μm is formed on a surface of the alloy.
 6. The austenitic heatresistant alloy according to claim 5, wherein in the case where thealloy is heated in the atmosphere containing steam at 900° C. for 20hours and subsequently heated in the H₂—CH₄—CO₂ atmosphere at 1100° C.for 96 hours, a layer having a Cr—Mn-based spinel structure formed onthe alumina layer has a thickness of 5 μm or less.
 7. An austenitic heatresistant alloy having a chemical composition comprising, in masspercent: C: 0.03 to 0.25%; Si: 0.01 to 2.0%; Mn: 0.10 to 0.50%; P:0.030% or less; S: 0.010% or less; Cr: 13.0 to 30.0%; Ni: 25.0 to 45.0%;Al: 2.5 to 4.5%; Nb: 0.05 to 2.00%; N: 0.05% or less; Ti: 0 to 0.20%; W:0 to 6.0%; Mo: 0 to 4.0%; Zr: 0 to 0.10%; B: 0 to 0.0100%; Cu: 0 to5.0%; REM: 0 to 0.10%; Ca: 0 to 0.050%; Mg: 0 to 0.050%; and thebalance: Fe and impurities.